It is known in the art to prepare an unsaturated diester by the catalytic oxidative carbonylation of a diolefin. More particularly, it is known to synthesize diesters by reacting carbon monoxide, oxygen and a diolefin such as 1,3-butadiene, isoprene, chloroprene, 2,3-dimethylbutadiene, 1,3-pentadiene and the like, under elevated temperature and pressure conditions in the presence of a catalytic amount of a ruthenium, rhodium, palladium, osmium, iridium or platinum metal salt compound of mixtures thereof, a copper (I), copper (II), iron (II) or iron (III) oxidant salt compound and a stoichiometric amount of a dehydrating agent which may be, for example, an orthoester, ketal, acetal, or trialkyl orthoborate. Co-catalytic ligands or coordination complex compounds of the metal salt compounds and catalytic quantities of a primary, secondary or tertiary saturated alcohol, while not required in this prior art process, may also be employed.
In the aforesaid process, the oxycarbonylation catalyst markedly deactivates after but two uses. The reason for deactivation is now thought to be the formation of dimethyl oxalate as a non-selective product in the oxycarbonylation reaction. Hydrolysis of the dimethyl oxalate by water formed during the reoxidation of copper (I) to copper (II) or iron (II) to iron (III) forms oxalic acid. Oxalic acid complexes with the soluble copper (II) or iron (III) reoxidant resulting in the formation of insoluble and inactive copper (II) or iron (III) oxalate. It is felt that because of copper (II) or iron (III) losses associated with the formation of copper (II) or iron (III) oxalate, the reoxidation of the platinum group metal such as palladium (O) by copper (II) or iron (III), is incomplete or slow resulting in agglomeration of the platinum group metal as well as a general loss in catalyst activity.